1. Field of the Invention
This invention relates generally to cardiac stimulator leads, and more particularly to a cardiac stimulator lead having a bent insulating sleeve and a shape memory torque coil.
2. Description of the Related Art
In a typical endocardial implantation procedure, the distal end of a cardiac lead is inserted through an incision in the chest and manipulated by the physician to the site requiring electrical stimulation with the aid of a flexible stylet that is removed prior to closure. At the site requiring electrical stimulation, the distal end of the lead is anchored to the endocardium by an active mechanism, such as a screw-in electrode tip, or alternatively, by a passive mechanism, such as one or more radially spaced tines that engage the endocardium. The proximal end of the lead is then connected to the cardiac pacemaker or defibrillator and the incision is closed. The implantation route and site are usually imaged in real time by fluoroscopy to confirm proper manipulation and placement of the lead.
A conventional cardiac stimulator lead normally consists of an elongated flexible tubular, electrically insulating sleeve that is connected proximally to a connector that is adapted to couple to the header of a cardiac pacemaker or defibrillator, and distally to a tubular tip electrode. One or more ring-type electrodes may be secured to the sleeve at various positions along the length of the sleeve. The proximal end of the lead sleeve is connected to the connector by application of various biocompatible adhesives to various portions of the connector and the sleeve. The tip electrode ordinarily consists of a tubular structure that has an increased diameter portion that forms an annular shoulder against which the distal end of the lead sleeve is abutted. The exterior surface of the tubular structure is normally smooth as is the interior surface of the distal end of the lead sleeve.
In conventional active fixation tip electrodes, engagement with the endocardium is often achieved by twisting a corkscrew that is attached to the tip electrode into the endocardium. In many conventional designs, the corkscrew is fixed to the tip electrode and the entire lead is twisted to screw the corkscrew into the heart tissue. In another common design, the corkscrew is projectable from the tip electrode. In some of these types of designs, the corkscrew is twisted with the aid of a stylet that is inserted into the lumen of the lead. In others, a coiled wire is positioned inside the lead, and connected distally to the corkscrew and proximally to a rotatable pin. As the pin is twisted by the physician, the coiled wire transmits the applied torque from the pin to rotate the corkscrew.
Conventional torque coils commonly consist of a strand of drawn wire that has been coiled into an elongated helix. At the time the wire is drawn and coiled, the torque coil takes on a straight-set, that is, the torque coil resembles a helical spring. In cardiac leads, such torque coils are commonly constructed of biocompatible metallic materials, such as MP35N or 316L stainless steel. The straight-set torque coil is then positioned inside an initially straight length of thermally setting tubing, such as polyurethane, and connected with the various other components of the lead. The lead is then wrapped around a mandrel or some other type of jig to establish the requisite J-shape or bend and heated above a given temperature for a set time to thermally set the bent shape.
The thermal setting of the lead sleeve to establish the requisite J or bent shape normally does not effect the memory of the torque coil, as the thermal setting is carried out at a temperature that is significantly lower than the temperatures that would cause grain structure changes in the material of the torque coil. However, the bending of the lead sleeve, and thus the torque coil, does result in what is in essence a cold working of the torque coil. As a result of this cold working, the torque coil loses its straight-set memory and takes on a bent set.
With a bent-set, conventional torque coils frequently cause the tips of conventional bent leads to move unpredictably when the physician twists the torque coil during implantation. The bent portion of the torque coil will cause the tip of the lead to precess as the torque coil is twisted proximally by the physician. The movement of the tip of the lead is not unlike the precession of the axis of rotation of a spinning gyroscope, albeit in a much more erratic and unpredictable fashion. As a consequence, the physician may accurately position the tip of the lead proximate the site requiring electrical stimulation only to have the tip migrate away from the desired location after the torque coil is twisted. This difficulty can lead to a series of trial and error attempts by the physician in order to obtain the proper positioning of the lead tip. In order to compensate for the erratic and unpredictable movement of the lead tip, physicians sometimes resort to first positioning the tip near the targeted tissue and then jabbing the tip into the heart wall. The physician then relies on irregular heart tissue, such as musculi pectinati, to prevent the tip from flopping about while the torque coil is twisted. This technique may also involve some trial and error and may not work well where fixation to smooth tissue is indicated.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.